1. Field
The present invention generally relates to techniques for charging a battery. More specifically, the present invention relates to a method and apparatus for charging a lithium-ion battery which adaptively controls the lithium surface concentration to remain within set limits.
2. Related Art
Rechargeable lithium-ion batteries are presently used to provide power in a wide variety of systems, including laptop computers, cordless power tools and electric vehicles. FIG. 1 illustrates a typical lithium-ion battery cell, which includes a porous graphite electrode, a polymer separator impregnated with electrolyte, and a porous cobalt dioxide electrode. The details of the transport of lithium and lithium ions in and out of the electrode granules and through the material between them are complex, but the net effect is dominated by slow diffusion processes for filling one electrode with lithium while removing it from the other.
Note that FIG. 1 provides a physical model for the layout of a typical lithium-ion cell, wherein the oxidation and reduction processes that occur during charging are also illustrated. The physical model shows the current collectors, which are in turn connected to the battery terminals; the polymer separator; and the positive and negative porous electrodes. Note that an electrolyte permeates the porous electrodes and the separator.
The negative electrode includes granules of graphite held together with a conductive binder (in practice, there may also be a nonconductive binder). Surrounding each graphite particle is a thin passivating layer called the solid-electrolyte interphase (SEI) that forms when a fresh cell is charged for the first time from the lithium atoms in the graphite reacting directly with the electrolyte. This occurs because the tendency for the lithium atoms to remain in the graphite is relatively weak when the cell is fully charged, but after the SEI is formed, the SEI acts as a barrier against further reactions with the electrolyte. Nevertheless, the SEI still allows transport of lithium ions, albeit with some degree of extra resistance.
The positive electrode includes granules of lithiated cobalt dioxide held together with binders similar to the negative electrode. Any SEI-like layer surrounding these particles is likely to be of much less significance than in the negative electrode because lithium atoms strongly favor remaining in these particles rather than leaving and reacting directly with the electrolyte.
Lithium transport in the negative graphite electrode (also referred to as the “transport-limiting electrode”) is slower than in the positive cobalt dioxide electrode (also referred to as the “non-transport-limiting electrode”), and therefore limits the maximal speed of charging. During charging, the slow diffusion causes a transient build-up of lithium on the surfaces of the graphite that varies in direct proportion to the charging current and a characteristic diffusion time.
The diffusion time is typically on the order of hours and has a strong dependence on temperature and other variables. For instance, a cell at 15° C. can have a diffusion time which is ten times slower than a cell at 35° C. The diffusion time can also vary significantly between cells, even under the same environmental conditions, due to manufacturing variability.
If the concentration of lithium at the surface reaches the saturation concentration for lithium in graphite, more lithium is prevented from entering the graphite electrode until the concentration decreases. A primary goal of conventional battery-charging techniques is to avoid lithium surface saturation, while keeping the charging time to a minimum. For example, one conventional technique charges at a constant current until a fixed upper voltage limit (e.g., 4.2 V) is reached, and then charges by holding the voltage constant at this upper limit until the current tapers to some lower limit. Note that it is common practice to express all currents in terms of the cell capacity. For example, for a cell with a capacity of Qmax=2500 mA·hr, a “1 C” current would be 2500 mA. In these units, the constant current charging is usually done at less than 1 C (e.g., 0.3 C), and the constant voltage phase is terminated when the current tapers to some value less than 0.05 C.
FIG. 2 illustrates a representative conventional charging profile. The problem with a conventional charging scheme is that it largely operates blindly; the only information used is the cell voltage, which does not directly correlate to the lithium surface concentration. Consequently, conventional charging both misses the opportunity to use more current when it is possible to do so, and enters the saturation region if lithium transport is slower than expected.
Hence, what is needed is a method and an apparatus for charging a lithium-ion battery that does not suffer from the drawbacks of these existing techniques.